There has been interest for many years in the development of a leadless glaze with a low melting temperature, a low viscosity, a wide firing range, and a high index of refraction characteristic of lead-containing glazes. There are several reasons for this interest. Occasional episodes of lead toxication have resulted from the use of improperly fired and formulated lead-containing glazes on ceramic ware. While it is easy to prepare lead-containing glazes which are safe (and, in fact, some of the most resistant glazes contain lead), the problem of lead toxication is avoided completely through the use of leadless glazes. In addition, limits on the lead content of waste water dumped into rivers and streams have been enacted in most states. Moreover, recent governmental regulations mandate reductions in blood lead levels for workers exposed to lead. These regulations require installation of costly new equipment by those who utilize lead-containing glazes. This expense would be avoided completely through the use of a leadless glaze. Finally, since lead-containing glazes are considerably more corrosive to kiln refractories than leadless glazes, an extension in kiln life could be expected through the use of a leadless glaze.
A number of leadless glazes have been known for many years. Glazes which are fired in excess of 1250.degree. F. commonly have been leadless. This is because glazes containing lead break down above 1250.degree. C. with excessive volatilization of lead oxide. These high-temperature glazes are used on sanitary ware and on hard paste porcelain. These leadless glazes are not, however, suitable for use on most of dinnerware, tile, and artware because of the very high maturation temperature.
Complex alkali, alkaline earth, boroaluminosilicate glazes have been developed, which in the laboratory can be successfully applied to dinnerware-type bodies. However, when use of these glazes is attempted on a commercial scale, defects are often encountered. In addition, glazes for semivitreous earthenware which combine these complex glazes with high alkali porcelain glazes have been developed. These glazes, however, have been unsuitable for vitreous dinnerware, artware and tile because of their high coefficient of thermal expansion which produces crazing on the substrate used.
The use of zinc oxide in glazes is also well known. In small amounts, it is a very active flux, although when added in larger quantities it may produce a matte. It is seldom used below 950.degree. C. because at the lower temperature it does not have much fluxing power. In conjunction with other fluxes, such as alkalis and boric acid, zinc is a very valuable material, contributing to the creation of a smooth, defect-free glaze surface. Conversely, when used in very large quantities, zinc oxide may lead to crawling, pitting and pinholing. However, there is one major problem with the use of zinc. In glazes of the prior art, zinc is known to have a profound effect on colors obtained from the various pigmenting materials. Many such pigmenting materials cannot be used. As a result, while zinc has been used in glazes for sanitary ware and tile where a limited palette of color is acceptable, it has not been used in dinnerware or artware glazes where a full palette of colors is required. Zinc has also not been used in decorative tile where the full palette of color is required.